Color printing by an electrophotographic printer is achieved by scanning a digitized image onto a photoconductor. Typically, the scanning is performed with laser diodes which pulse a beam of laser energy onto the photoconductor. Light emitting diodes (LEDs) can be used in place of the laser diodes. The photoconductor typically comprises a drum or a belt coated with a photoconductive material capable of retaining localized electrical charges. Each localized area capable of receiving a charge corresponds to a pixel. Each pixel is charged to a base electrical charge, and then is either exposed or not exposed by the laser, as dictated by the digital data used to pulse the laser. Exposing a pixel corresponds to electrically altering (typically discharging) the localized area from the base electrical charge to a different electrical charge. One charge will attract toner, and the other charge will not. In this manner, toner is selectively transferred to the photoconductor. In most electrophotographic printing processes, the exposed (electrically discharged) pixels attract toner onto the photoconductor. This process is known as discharge area development (DAD). However, in some electrophotographic printing processes the toner is attracted to the un-discharged (i.e., charged) area on the photoconductor. This latter type of electrophotographic printing is known as charge-area-development (CAD). For purposes of discussion, it will be assumed that DAD is used, although the present invention is not limited to DAD.
Once the photoconductor has had the desired toner transferred to it, the toner is then transferred to the intermediate or finished product medium. This transfer can either be direct or it can be indirect using an intermediate transfer device. The finished product medium typically comprises a sheet of paper, normally white, but can also comprise a transparency or a colored sheet of paper. After the toner is transferred to the finished product medium, it is processed to fix the toner to the medium. This last step is normally accomplished by thermally heating the toner to fuse it to the medium, or applying pressure to the toner on the medium.
There are a variety of known methods for selectively attracting toner to a photoconductor. Generally, each toner has a known electrical potential affinity. Selected areas of the photoconductor are exposed from a base potential to the potential for the selected toner, and then the photoconductor is exposed to the toner so that the toner is attracted to the selectively exposed areas. This latter step is known as developing the photoconductor. In some processes, after the photoconductor is developed by a first toner, the photoconductor is then recharged to the base potential and subsequently exposed and developed by a second toner. In other processes, the photoconductor is not recharged to the base potential after being exposed and developed by a selected toner. In yet another process, the photoconductor is exposed and developed by a plurality of toners, then recharged, and then exposed and developed by another toner. In certain processes, individual photoconductors are individually developed with a dedicated color, and then the toner is transferred from the various photoconductors to a transfer medium which then transfers the toner to the finished product medium. The selection of the charge-expose-develop process depends on a number of variables, such as the type of toner used and the ultimate quality of the image desired. The quality of the final image on the medium is typically associated with complexity and cost of the printer, such that higher quality electrophotographic printers which produce higher quality images are more complex, and concomitantly more expensive.
Image data for an electrophotographic printer (which will also be known herein as a "laser printer"), including color laser printers, is digital data which is stored in computer memory. The data is stored in a matrix or "raster" which identifies the location and color of each pixel which comprises the overall image. The raster image data can be obtained by scanning an original analog document and digitizing the image into raster data, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using a printer. Accordingly, the technology to which the invention described below is applicable to either photocopiers or printers. Recent technology has removed this distinction, such that a single printing apparatus can be used either as a copier or as a printer for computer files. These apparatus have been known as multifunction printers ("MFPs)", a term indicating the ability to act as a photocopier, a printer, or a facsimile machine. In any event, the image to be printed onto tangible media is stored as a digital image file. The digital image data is then used to pulse the beam of a laser in the manner described above so that the image can be reproduced by the electrophotographic printing apparatus. Accordingly, the expression "printer" should not be considered as limiting to a device for printing a file from a computer, but should also include a photocopier capable of printing a digitized image of an original document. "Original documents" include not only already digitized documents such as text and image files, but photographs and other images, including hybrid text-image documents, which are scanned and digitized into raster data.
The raster image data file is essentially organized into a two dimensional matrix. The image is digitized into a number of lines. Each line comprises a number of discrete dots or pixels across the line. Each pixel is assigned a binary value relating information pertaining to its color and potentially other attributes, such as density. The combination of lines and pixels makes up the resultant image. The digital image is stored in computer readable memory as a raster image. That is, the image is cataloged by line, and each line is cataloged by each pixel in the line. A computer processor reads the raster image data line by line, and actuates the laser to selectively expose a pixel based on the presence or absence of coloration, and the degree of coloration for the pixel. Typical pixel densities for images are in the range of 300 to 1200 pixels per inch, in each direction.
The method of transferring the digital raster data to the photoconductor via a laser, lasers or LEDs is known as the image scanning process or the scanning process. The scanning process is performed by a scanning portion or scanning section of the electrophotographic printer. The process of attracting toner to the photoconductor is known as the developing process. The developing process is accomplished by the developer section of the printer. Image quality is dependent on both of these processes. Image quality is thus dependent on both the scanning section of the printer, which transfers the raster data image to the photoconductor, as well as the developer section of the printer, which manages the transfer of the toner to the photoconductor.
A typical in-line color laser printer utilizes a plurality (typically 4) laser scanners to generate a latent electrostatic image for each color plane to be printed. This allows for four colors to be imaged on a photoconductor in a single pass of the photoconductor past the laser. Alternately, a single laser can be used and the photoconductor passed by the laser four times. This latter method is known as four-pass color printing. While four-pass color printing allows a single laser diode to be used and thus provides for a simplified design over in-line imaging, it is essentially four times slower than in-line imaging.
The four color planes typically printed, and which are generally considered as necessary to generate a relatively complete palate of colors, are yellow, magenta, cyan and black. That is, the color printer is typically provided with toners in each of these four colors. These colors will be known herein as the "primary colors". Some printers have the capability of printing one base color on top of another on the same pixel, so as to generate a fuller palate of finished colors. However, this normally requires recharging the photoconductor between developing stations and performing a secondary exposure through already deposited toner. The subsequent exposure on top of already deposited toner normally does not acquire the same electrical properties as an exposure of the photoconductor where no toner has been deposited. This results in uneven distribution of toner between secondary developed pixels and originally developed pixels. Alternately, four photoconductors, one for each primary color, can be used in conjunction with an intermediate transfer belt. This configuration is described more fully below with respect to the prior art apparatus shown in FIG. 1. While a multiple photoconductor configuration resolves the problem of developing subsequent toner over already developed toner, it adds a significant degree of complexity to the printing apparatus.
In the scanning process, a laser is scanned from one edge of the photoconductor to the opposing edge and is selectively actuated or not actuated on a pixel-by-pixel basis to scan a line of the image onto the photoconductor. The photoconductor advances and the next line of the image is scanned by the laser onto the photoconductor. In a multiple laser printer, more than one laser can be actuated simultaneously so as to more quickly generate the complete image onto the photoconductor. The side-to-side scanning of each laser is traditionally accomplished using a dedicated multi-sided or faceted rotating mirror. Such a mirror will be known herein as a "polygon" due to the polygonal shape of the mirror. The reflective surface of the mirrors are typically ground and polished aluminum. The laser beam impinges on one facet of the polygonal mirror and is reflected to a secondary or deflector mirror, which directs the laser beam to a unique, relative lineal position on the light sensitive surface of the photoconductor. By "relative", it is understood that the photoconductor moves with respect to the linear position, but the position remains fixed in space. As the polygonal mirror rotates, the angle of incidence, and hence the angle of reflection, of the laser beam will vary. This causes the laser beam to be scanned across the photoconductor at the unique relative lineal position from a first edge to a second edge of the photoconductor. As the mirror rotates to an edge of the polygon between facets, the laser is essentially reset to the first edge of the photoconductor to begin scanning a new line onto the advancing photoconductor. These mirrors tend to rotate at very high speeds, often in excess of 20,000 rpm.
For color printing, it is important to assure the registration of the different colors. That is, each laser should be aligned with respect to the other lasers such that a given pixel in the raster image is associated with a single common point on the photoconductor, regardless of which laser is used to identify the point. A registration which is "off" will result in a blurry image, or an image with colors not representative of the raster image. Registration is thus dependent on aligning all of the lasers in a laser printer. Each laser and its associated components (i.e., rotating mirror, optical elements, and deflector mirror) are typically mounted in a precision housing to keep the components in relative fixed position with respect to one another. The housings are typically molded plastic. Assuring registration of the lasers requires aligning the four housings within the printer itself. As environmental conditions within the printer change (e.g., temperature), this alignment can change.
FIG. 1 depicts a schematic side elevation diagram of a prior art four laser color electrophotographic printer "A". The printer "A" comprises a scanning section "B" and a photoconductor section "C". The photoconductor section shown here comprises a rotating belt 5 which supports a photoconductive material. Four developing stations, 6, 7, 8 and 9, are located proximate to the belt 5 and affix toner to the photoconductor in response to selective exposure of the photoconductive material by the laser beams at points "D", "E", "F" and "G" along the belt. For exemplary purposes only, developing station 6 can be the yellow developer, station 7 can be the magenta developer, station 8 can be the cyan developer, and station 9 can be the black developer. Each developing station has its own photoconductor, base charging unit to apply a base electrical charge to the photoconductor, and cleaning station to remove surplus toner from the photoconductor. Toner applied to each photoconductor is in turn transferred to the transfer belt 5. Prior to each developing station is a base charging unit 23 which applies a base electrical potential or bias to the transfer belt 5. In this way, the transfer belt is charged, a first color is applied to the transfer belt, the belt is recharged, a second color is applied, and so on. Finally, all four colors are transferred from the transfer belt 5 to the finished product medium "I" at transfer station 24. Any residual toner is removed from the transfer belt at belt cleaning station 25.
The scanning section "B" in FIG. 1 comprises four scanning laser stations, 11, 12, 13 and 14. Each scanning station comprises a laser 15, a rotating mirror 16, a motor 17 for driving the mirror 16, a laser beam focusing lens 19, an aligning lens 18, a deflector mirror 21 for deflecting the laser beam onto the photoconductor belt 5, and a housing 22 for holding the aforementioned components.
Since only partial alignment of the laser beams with respect to one another can be achieve by aligning the housings which contain the scanning assemblies, in-line color printers are typically also provided with color plane sensors to sense color plane alignment. Sensors are provided to detect shifts in color planes in both the side-to-side scanning direction (the "scan" direction), as well as in the direction of advance of the photoconductor (i.e., the "process" direction). The sensors can provide a feedback to the scanning system and corrections can be made to reposition the laser beam using various known electrical and mechanical methods.
The space required within a printer unit for a plurality of scanning assembly housings tends to reduce the focal length which can be achieved with each laser (i.e., the distance from the focusing lens to the photoconductor surface). Generally, shorter focal lengths require higher quality optics to focus the beam over the shorter distances. Obtaining greater focal lengths with multiple scanning assemblies would require increasing the size of the printer. Since many printers are chosen for desk-top use, a large printer is undesirable.
Each rotating mirror assembly is driven by its own dedicated motor. The power consumption for each mirror driving motor is typically about 20 watts. Thus, for a four-laser printer, the mirror drives alone consume about 80 watts. This requires a larger power supply, generates a fair amount of heat, and generally adds cost and complexity to the overall printer design.
What is needed then is an electrophotographic color printer which reduces the complexity of the scanning section.
The developing section of electrophotographic color printers is another area where simplification is desirable. The developer sections of most electrophotographic printers require that the photoconductive material be recharged between developers. In a four color printer using one-pass imaging, this requires three additional charging stations, which typically comprise a corona discharge device which is frequently a thin wire held in close proximity to the photoconductor. Over time, these wires need to be replaced. Other designs comprise four separate photoconductor substations which each individually deposit toner of a unique color onto a transfer belt. The transfer belt then redeposits the toner onto the print media. This latter configuration overcomes the difficulty of printing color-on-color, yet results in a complex apparatus for the developer section. Specifically, each photoconductor substation is provided with its own photoconductor, charging device, transfer mechanism to transfer toner to the transfer belt, and cleaning station to remove residual toner from the photoconductor. In addition, the transfer belt is provided with a charging station and a cleaning station. For four-color printing, the number of components in this latter configuration is quite large, and the power requirements become significant as well.
In addition to problems of complexity in the prior art developer sections, producing quality color images is also a challenge. Specifically, there are numerous problems in getting toner to deposit on the photoconductor or transfer belt in a manner which faithfully reproduces the raster data image. Some of those problems will now be discussed.
When a single photoconductor is used to collect the various toners prior to depositing them on the final print media, an exposed area is typically left unsaturated. That is, the toner does not completely electrically saturate the exposed area to bring the electrical field on the area to zero. This leaves a residual electrical field on the photoconductor which can attract toner from the next developer station onto the already deposited toner. Ideally, toner develops to completion (i.e., a sufficient amount of charged toner builds up in the exposed area such that the voltage of the developed area equals the voltage bias of the developer and the electrical field goes to zero); in reality, this typically does not occur. Frequently the exposed field is only developed to fifty or sixty percent of completion. Charging between developments helps to overcome this problem, but requires three additional charging stations, as discussed previously.
The single-photoconductor configuration also allows previously deposited toner to migrate into the toner hoppers of subsequent toners. This is a result of recharging the developer between developer substations. The recharge not only charges the photoconductive material, but the previously deposited toner. When provided with an electrical field, the toner can actually leave the photoconductor and migrate into subsequent toner hoppers, thereby contaminating the toner for subsequent printing.
What is needed then is a developing process and apparatus which allows the raster image to be more accurately printed onto the photoconductor by decreasing contamination of already developed pixels with subsequent toner, and reduces contamination of toner hoppers with already deposited toner.